EP4191878A1 - Bulk acoustic wave device and method for manufacturing such a device - Google Patents

Abstract

The invention relates to an electro-acoustic device (1) comprising, stacked along a direction (z), a silicon-based substrate (10), a first electrode (21), a piezoelectric layer (30) based on a perovskite taken from lithium niobate LiNbO3, lithium tantalate LiTaO3, or an alloy Li(Nb,Ta)O3, on the first electrode (21), a second electrode (22) arranged on the piezoelectric layer (30) . Advantageously, the first electrode (21) is made of an electrically conductive refractory material based on nitride, such as TiN, VN, TaN. Said electro-acoustic device is a bulk acoustic wave filter guided on a Bragg reflector, comprising, between the substrate (10) and the first electrode (21), a stack (50) of layers (51, 52) presenting alternately high and low acoustic impedances. The low impedance layers (52) are based on TiN or VN, and the high impedance layers (51) are based on HfN or TaN. (1).

Description

TECHNICAL AREA

The present invention relates, in general, to a bulk acoustic wave device, and more particularly, to a method for producing the layer based on a piezoelectric material forming the core of this device.

STATE OF THE ART

Resonators based on a “bulk acoustic wave” type structure known as BAW (acronym for “Bulk Acoustic Wave”) are commonly used to produce RF filters. The core of the BAW resonators is composed of a piezoelectric material which influences the final properties of the filter.

Lithium niobate (LiNbO3) has been used for several years as a piezoelectric material. Its intrinsic properties, such as its piezoelectric coupling coefficient, could allow the filter to resonate at high frequencies, for example at frequencies above 3.5 GHz, with a bandwidth of up to 600 MHz. To obtain a BAW filter having a high coupling coefficient, typically up to 50%, the piezoelectric layer based on LiNbO3 must in particular have a high crystalline quality and a controlled stoichiometry.

The document " A. Reinhardt et al., Acoustic filters based on thin single crystal LiNbO3 films: Status and prospects Proc. - IEEE Ultrasound. Symp. 2014, 773 » discloses a solution based on a transfer process known under the name Smart-Cut ^™ making it possible to manufacture a BAW filter comprising a piezoelectric layer based on LiNbO3 on a silicon substrate. A thin layer of LiNbO3 is taken from a donor substrate, typically from a monocrystalline LiNbO3 substrate, then transferred to a receiver substrate, typically a silicon substrate. This solution implements many technical steps and has a significant cost.

Another solution consists in synthesizing the piezoelectric layer based on LiNbO3 on the silicon substrate, via a buffer layer. The document " Growth of highly near-c-axis oriented ferroelectric LiNbO3 thin films on Si with a ZnO buffer layer, P. You et al., Appl. Phys. Lett. 2013, 102, 051914 proposes forming a buffer layer based on zinc oxide on the silicon substrate, before forming the layer based on lithium niobate on this buffer layer. The piezoelectric layer based on LiNbO3 thus synthesized nevertheless presents a texturing. It is not monocrystalline. The ZnO-based buffer layer is also not completely compatible with integration within a functional device, in particular with certain etching steps such as etching of the lower electrode or etching for the formation of the cavity under the ZnO. Problems adherence are also observed.

There is therefore a need consisting in producing an electro-acoustic device comprising a piezoelectric layer based on lithium niobate on a substrate based on silicon, which has improved quality, while limiting the production costs.

An objective of the present invention is to meet this need at least in part.

The other objects, features and advantages of the present invention will become apparent from a review of the following description and the accompanying drawings. It is understood that other benefits may be incorporated.

SUMMARY

• Fournir un substrat à base de silicium,
• Former une première électrode sur le substrat,
• Former la couche piézoélectrique sur la première électrode,
• Former une deuxième électrode sur la couche piézoélectrique.

To achieve this objective, according to one embodiment, a method is provided for producing an electro-acoustic device comprising a piezoelectric layer, said method comprising the following steps:

• Provide a silicon-based substrate,
• Form a first electrode on the substrate,
• Form the piezoelectric layer on the first electrode,
• Form a second electrode on the piezoelectric layer.

Advantageously, the formation of the piezoelectric layer takes place by epitaxy, on said first electrode, of a material of the ABO3 type, O being oxygen, A being at least one first chemical element taken from sodium (Na), potassium (K), barium (Ba), lithium (Li), lead (Pb), and B being at least a second chemical element taken from zirconium (Zr), titanium (Ti), niobium (Nb) , tantalum (Ta).

Advantageously, the first electrode is chosen from an electrically conductive refractory material based on nitride, such as TiN, VN, ZrN, TaN, HfN, NbN, or an alloy of said materials.

Preferably, the ABO3 material of the piezoelectric layer is taken from lithium niobate LiNbO3, lithium tantalate LiTaO3, or a Li(Nb,Ta)O3 alloy.

• Avoir une structure cristalline et/ou un paramètre de maille compatible avec un substrat à base de silicium. Cela permet par exemple d'épitaxier la première électrode sur le substrat.
• Avoir une structure cristalline et/ou un paramètre de maille proche ou compatible avec le niobate de lithium ou le tantalate de lithium. Cela permet d'épitaxier la couche piézoélectrique sur la première électrode, qui forme ainsi une couche de nucléation.
• Avoir un coefficient de dilatation thermique proche du niobate de lithium ou du tantalate de lithium. Cela permet de limiter l'apparition de fissures due aux variations de températures lors de la formation de la couche piézoélectrique.
• Etre électriquement conductrice.
• Pouvoir bloquer la diffusion des atomes de Li dans le Si. Cela permet d'éviter une perte de stœchiométrie d'une couche piézoélectrique à base de niobate de lithium ou de tantalate de lithium, dite couche LN/LT.
• Confiner une onde acoustique, notamment en vue de garantir de hautes performances pour les dispositifs électro-acoustiques.

Within the framework of the development of the present invention, certain conditions for the growth by epitaxy of the piezoelectric layer have been determined. The first electrode should preferably have at least some of the following properties:

• Have a crystal structure and/or lattice parameter compatible with a silicon-based substrate. This makes it possible, for example, to epitaxy the first electrode on the substrate.
• Have a crystal structure and/or lattice parameter close to or compatible with lithium niobate or lithium tantalate. This makes it possible to epitax the piezoelectric layer on the first electrode, which thus forms a nucleation layer.
• Have a coefficient of thermal expansion close to lithium niobate or lithium tantalate. This makes it possible to limit the appearance of cracks due to temperature variations during the formation of the piezoelectric layer.
• Be electrically conductive.
• To be able to block the diffusion of Li atoms in Si. This makes it possible to avoid a loss of stoichiometry of a piezoelectric layer based on lithium niobate or lithium tantalate, known as the LN/LT layer.
• Confining an acoustic wave, in particular with a view to guaranteeing high performance for electro-acoustic devices.

A technical prejudice of the state of the art is that the nucleation layer must necessarily be based on oxide in order to avoid a loss of stoichiometry of the LN/LT layer due to a diffusion of oxygen from the LN layer. /LT to the nucleation layer.

On the contrary, to meet these specifications, the first electrode forming the nucleation layer is chosen according to the present invention in an electrically conductive refractory material based on nitride. This material is typically a refractory transition metal-based nitride, for example TiN, VN, ZrN, TaN, HfN, NbN, or their alloys. In the context of the development of the present invention, it has in fact been observed that such a nucleation layer allows, in a perfectly unexpected manner, the epitaxy of the LN/LT layer under good conditions. Surprisingly, it has also been observed that such a nucleation layer makes it possible to block both the diffusion of oxygen and the diffusion of lithium towards the silicon-based substrate. As a result, the LN/LT layer epitaxied on such a nucleation layer retains the required stoichiometry.

Furthermore, the different crystalline structures and the lattice parameters of the refractory transition nitrides are perfectly compatible with those of the LN/LT materials and of silicon. Such a nucleation layer can therefore advantageously be epitaxially grown on a silicon-based substrate, then allow the epitaxy of the LN/LT layer.

Furthermore, refractory transition nitrides are conductive and can therefore be used as the first electrode of a BAW filter. If these materials are epitaxied on silicon, they can present resistivity values of the order of 15 μOhm.cm, which is perfectly compatible with the manufacture of a BAW filter.

Furthermore, refractory transition nitrides are resistant to the growth conditions of LiNbO3, in particular the high temperature required for the growth of LiNbO3, which can be of the order of 1000°C.

Furthermore, refractory transition nitrides are hard enough to protect the silicon surface during the growth of LiNbO3. For example, they protect the silicon from the plasma and from the impact of the ions used in the implementation of a PVD physical vapor deposition technique for the growth of LiNbO3.

Refractory transition nitrides can also have interesting rigidity and acoustic impedance values to further confine the acoustic waves in the piezoelectric layer.

The present invention thus proposes a solution making it possible to form an electro-acoustic device on a silicon-based substrate - without a transfer step - by means of the first electrode based on a refractory transition nitride, playing the role of nucleation layer. Thanks to such a nucleation layer in an electrically conductive refractory material based on nitride, the piezoelectric layer LN/LT is stoichiometric and of high crystalline quality.

The piezoelectric layer thus has the properties required for a high-frequency BAW filter type application.

• un substrat à base de silicium,
• une première électrode sur ledit substrat,
• une couche piézoélectrique sur ladite première électrode, ladite couche piézoélectrique étant à base d'un matériau du type ABO3, O étant l'oxygène, A étant au moins un premier élément chimique pris parmi le sodium (Na), le potassium (K), le baryum (Ba), le lithium (Li), le plomb (Pb), et B étant au moins un deuxième élément chimique pris parmi le zirconium (Zr), le titane (Ti), le niobium (Nb), le tantale (Ta),
• une deuxième électrode disposée sur la couche piézoélectrique.

According to another aspect of the invention, an electro-acoustic device is provided, in particular a bulk acoustic wave (BAW) device, comprising, stacked in a vertical direction:

• a silicon-based substrate,
• a first electrode on said substrate,
• a piezoelectric layer on said first electrode, said piezoelectric layer being based on a material of the ABO3 type, O being oxygen, A being at least one first chemical element taken from sodium (Na), potassium (K), barium (Ba), lithium (Li), lead (Pb), and B being at least a second chemical element taken from zirconium (Zr), titanium (Ti), niobium (Nb), tantalum ( Your),
• a second electrode disposed on the piezoelectric layer.

Advantageously, the first electrode of the BAW device is made of a material Nitride-based electrically conductive refractory. Preferably, the piezoelectric layer is based on lithium niobate LiNbO3, or lithium tantalate LiTaO3, or an alloy Li(Nb,Ta)O3. The advantages mentioned above apply mutatis mutandis. Such a device can also be directly integrated in silicon technology. It can be easily co-integrated with other microelectronic or optoelectronic devices or even with (opto-)electromechanical microsystems (MEMS or MOEMS, respective acronyms for “Microelectromechanical systems” and “Microoptoelectromechanical systems”).

BRIEF DESCRIPTION OF FIGURES

• La figure 1 illustre en coupe transverse un dispositif électro-acoustique de type BAW selon un mode de réalisation la présente invention.
• La figure 2 illustre en coupe transverse un dispositif électro-acoustique de type BAW FBAR selon un mode de réalisation la présente invention.
• La figure 3A illustre en coupe transverse un dispositif électro-acoustique de type BAW SMR selon un mode de réalisation la présente invention.
• La figure 3B illustre en coupe transverse un dispositif électro-acoustique de type BAW SMR selon un autre mode de réalisation la présente invention.
• Les figures 4A à 4E illustrent des étapes d'un procédé de fabrication d'un dispositif électro-acoustique de type BAW FBAR selon un mode de réalisation la présente invention.

The aims, objects, as well as the characteristics and advantages of the invention will emerge better from the detailed description of embodiments of the latter which are illustrated by the following accompanying drawings in which:

• There figure 1 illustrates in transverse section an electro-acoustic device of the BAW type according to one embodiment of the present invention.
• There picture 2 illustrates in transverse section an electro-acoustic device of the BAW FBAR type according to one embodiment of the present invention.
• There Figure 3A illustrates in transverse section an electro-acoustic device of the BAW SMR type according to one embodiment of the present invention.
• There Figure 3B illustrates in transverse section an electro-acoustic device of the BAW SMR type according to another embodiment of the present invention.
• THE figures 4A to 4E illustrate steps of a method of manufacturing an electro-acoustic device of the BAW FBAR type according to one embodiment of the present invention.

The drawings are given by way of examples and do not limit the invention. They constitute schematic representations of principle intended to facilitate understanding of the invention and are not necessarily at the scale of practical applications. In particular, on the principle diagrams, the thicknesses of the various layers and portions, and the dimensions of the patterns are not representative of reality.

DETAILED DESCRIPTION

• Une résistance à l'oxydation,
• Une barrière à la diffusion du lithium.

Before starting a detailed review of embodiments of the invention, optional characteristics are set out below which may possibly be used in combination or alternatively:
According to one example, the nitride-based electrically conductive refractory material of the first electrode is chosen so as to present or form:

• Resistance to oxidation,
• A barrier to the diffusion of lithium.

This makes it possible to avoid a diffusion of oxygen and lithium from the piezoelectric layer LN/LT towards the first electrode. The stoichiometry of the LN/LT layer is thus preserved.

According to one example, the nitride-based refractory electrically conductive material of the first electrode is chosen so as to have a sufficiently high hardness or mechanical rigidity to make it possible to confine an acoustic wave in the piezoelectric layer LN/LT. For example, the Young's modulus of this material is greater than 300 GPa. Its hardness or resistance can be greater than 9000 MPa.

According to one example, the nitride-based refractory electrically conductive material of the first electrode is chosen so as to have a crystallographic structure compatible with the substrate and the piezoelectric layer LN/LT, such as a face-centered hexagonal or cubic structure. This makes it possible to limit the appearance of structural defects during the formation of the LN/LT layer by epitaxy.

According to one example, the formation of the first electrode is configured so that said first electrode has a thickness e2 of between 10 nm and 1000 nm, for example 100 nm. This makes it possible to obtain the desired crystalline quality and/or to limit the appearance of structural defects.

According to one example, the formation of the LN/LT piezoelectric layer is configured so that said LN/LT layer has, after epitaxy, a thickness e3 of between several hundreds of nanometers and several microns. Such a thickness e3 makes it possible to obtain the desired resonance frequency for the BAW filter.

According to one example, the refractory electrically conductive material based on nitride is taken from refractory transition nitrides based on a transition metal such as titanium nitride TiN, tantalum nitride TaN, niobium nitride NbN, zirconium nitride ZrN, hafnium nitride HfN, vanadium nitride VN, or their alloys.

According to one example, the nitride-based electrically conductive refractory material of the first electrode is chosen from a refractory transition nitride such as titanium nitride TiN, tantalum nitride TaN, niobium nitride NbN, zirconium nitride ZrN , hafnium nitride HfN, vanadium nitride VN, or their alloys.

According to one example, the electro-acoustic device is a bulk acoustic wave filter of the suspended membrane type. According to one example, the process comprises, after formation by epitaxy of the piezoelectric layer, an etching of the silicon-based substrate under the first electrode bearing the piezoelectric layer, so as to form a cavity.

According to one example, the electro-acoustic device is a guided volume acoustic wave filter on a Bragg reflector. According to one example, the method comprises, before formation of the first electrode, formation of a stack of layers having alternately high and low acoustic impedances.

According to one example, the low impedance layers are based on TiN or VN, and the high impedance layers are based on HfN or TaN.

According to one example, the formation of the first electrode and/or the formation of the piezoelectric layer LN/LT are carried out by pulsed laser ablation.

According to one example, the formation of the first electrode and the formation of the piezoelectric layer LN/LT are carried out by successively pulsed laser ablation within the same reactor without venting between said formations. This makes it possible to avoid an intermediate surface cleaning step, to reduce the total duration of the process and to limit the costs. This also makes it possible to obtain a low surface roughness of the LN/LT layer.

According to one example, the substrate is based on silicon oriented along (111), the first electrode is based on titanium nitride TiN oriented along (111), and the piezoelectric layer is oriented along (0001).

According to one example, the piezoelectric layer LN/LT is directly in contact with the first electrode. In particular, there is no intermediate oxide layer between the first electrode and the LN/LT layer.

According to one example, the refractory electrically conductive material based on nitride of the first electrode is a refractory transition nitride such as titanium nitride TiN, tantalum nitride TaN, niobium nitride NbN, zirconium nitride ZrN, hafnium nitride HfN, vanadium nitride VN, or their alloys.

According to one example, the electro-acoustic device is a bulk acoustic wave filter of the suspended membrane type, comprising a cavity under the first electrode bearing the piezoelectric layer.

According to one example, the electro-acoustic device is a bulk acoustic wave filter guided on a Bragg reflector, comprising, between the substrate and the first electrode, a stack of layers having alternately high and low acoustic impedances.

According to one example, the first and second electrodes are contacted at the level of a front face of the device.

According to one example, the first electrode is contacted at a rear face of the device, and the second electrode is contacted at a front face of the device.

According to one example, the silicon-based substrate is formed from a material chosen from among: silicon, SiC, SiGe.

According to one example, the silicon-based substrate is monocrystalline.

According to an alternative example, the silicon-based substrate is polycrystalline.

According to one example, the silicon-based substrate is oriented along (111).

Except incompatibility, it is understood that all of the optional characteristics above can be combined so as to form an embodiment which is not necessarily illustrated or described. Such an embodiment is obviously not excluded from the invention. The characteristics of one aspect of the invention, for example the device or the method, can be adapted mutatis mutandis to the other aspect of the invention.

It is specified that, in the context of the present invention, the terms "over", "overcomes", "covers", "underlying", "opposite" and their equivalents do not necessarily mean "over". contact of”. Thus, for example, the deposition of a first layer on a second layer does not necessarily mean that the two layers are directly in contact with each other, but means that the first layer at least partially covers the second layer in being either directly in contact with it, or by being separated from it by at least one other layer or at least one other element.

A layer can also be composed of several sub-layers of the same material or of different materials.

A substrate, a stack, a layer, "based" on a material A, is understood to mean a substrate, a stack, a layer comprising this material A only or this material A and possibly other materials, for example elements alloy and/or doping elements. Thus, a silicon-based substrate means for example an Si, doped Si, SiC, SiGe substrate. A TiN-based layer means for example a TiN layer, doped TiN, or TiN alloys.

The term "selective etching with respect to" or "etching having selectivity with respect to" an etching configured to remove a material A or a layer A with respect to a material B or d 'a layer B, and having an etching speed of material A greater than the etching speed of material B. The selectivity is the ratio of the etch rate of material A to the etch rate of material B.

“Electrical conductor” means a material having a resistivity value of less than 10,000 μOhm.cm, preferably less than 300 μOhm.cm, and even more preferably less than 100 μOhm.cm, in particular for an electrical conductor formed by a lower electrode.

Several embodiments of the invention implementing successive steps of the manufacturing method are described below. Unless explicitly mentioned, the adjective “successive” does not necessarily imply, even if this is generally preferred, that the stages follow each other immediately, intermediate stages possibly separating them.

Furthermore, the term “step” means the performance of part of the method, and can designate a set of sub-steps.

Moreover, the term "stage" does not necessarily mean that the actions carried out during a stage are simultaneous or immediately successive. Certain actions of a first step can in particular be followed by actions linked to a different step, and other actions of the first step can be resumed afterwards. Thus, the term step does not necessarily mean unitary and inseparable actions in time and in the sequence of the phases of the process.

A preferably orthonormal frame, comprising the axes x, y, z is shown in the appended figures. When only one reference is represented on the same sheet of figures, this reference applies to all the figures of this sheet.

In the present patent application, the thickness of a layer is taken along a direction normal to the main extension plane of the layer. Thus, a layer typically has a thickness along z. The relative terms “over”, “overcomes”, “under”, “underlying”, “intercalated” refer to positions taken along the z direction.

The terms “vertical”, “vertically” refer to a direction along z. The terms "horizontal", "horizontally", "lateral", "laterally" refer to a direction in the xy plane. Unless explicitly mentioned, the thickness, the height and the depth are measured according to z.

An element located " plumb " or " in line with " another element means that these two elements are both located on the same line perpendicular to a plane in which extends mainly a lower or upper face of a substrate, that is to say on the same line oriented vertically in the figures.

• le BAW-SMR (acronyme de « solidly mounted resonator »), qui comprend typiquement un empilement de couches présentant alternativement des impédances acoustiques fortes et faibles, l'ensemble jouant le rôle d'un réflecteur de Bragg ;
• le BAW-HBAR (acronyme de « High overtone Bulk Acoustic Resonator ») ; c'est le substrat lui-même qui est le lieu d'ondes stationnaires, typiquement sur plusieurs dizaines ou centaines de micromètres ;
• le BAW-FBAR (acronyme de « Film Bulk Acoustic Resonator ») ; c'est une membrane, séparée du substrat par une zone de vide, qui est en résonance.

In the context of the present invention, several electro-acoustic devices of the BAW type can be envisaged. These devices are for example:

• the BAW-SMR (acronym for “solidly mounted resonator”), which typically comprises a stack of layers presenting alternately high and low acoustic impedances, the assembly acting as a Bragg reflector;
• the BAW-HBAR (acronym for “High overtone Bulk Acoustic Resonator”); it is the substrate itself which is the site of standing waves, typically over several tens or hundreds of micrometers;
• the BAW-FBAR (acronym for “Film Bulk Acoustic Resonator”); it is a membrane, separated from the substrate by a vacuum zone, which is in resonance.

This list is not exhaustive. Other devices can be considered.

In the context of the present invention, a refractory transition nitride is a nitride of a transition metal or transition element (element whose atoms have an incomplete d electron sub-shell, or which can form cations whose sub- electron shell d is incomplete). These elements are grouped together in block d of the periodic table of elements.

In the examples described below, the LN/LT layer illustrated is based on lithium niobate LiNbO3. A layer of lithium tantalate LiTaO3 or a layer based on an alloy Li(Nb,Ta)O3 can be substituted for this layer of lithium niobate LiNbO3 within the scope of this invention. Thus, all the characteristics and all the technical effects mentioned about LiNbO3 are perfectly applicable and combinable with a layer of LiTaO3 or Li(Nb,Ta)O3.

X-ray diffraction analyses, for example in 2θ configuration, or in rotation according to ϕ and/or Ω (phi-scan and omega-scan), can be carried out in order to determine the crystalline quality of the LN/LT layers and of the electrodes or nucleation layers, and their epitaxial relationship.

There figure 1 illustrates in transverse section a device 1 of the BAW HBAR filter type, according to one embodiment of the invention. This device 1 typically comprises a substrate 10 based on silicon on which is formed a first electrode 21 based on titanium nitride TiN. A layer 30 of lithium niobate LiNbO3 is directly in contact with the first electrode 21. A second electrode 22 surmounts the piezoelectric layer 30 LN/LT.

The use of a first electrode based on electrically conductive refractory nitride, in particular TiN, advantageously makes it possible to carry out hetero-epitaxy of LiNbO3 on a silicon substrate. This first electrode 21 can thus be likened to a nucleation layer. This solution makes it possible to produce high-performance BAW electroacoustic devices 1 . Epitaxy notably makes it possible to obtain a layer of stoichiometric LiNbO3, preferentially oriented according to a single crystalline orientation in all directions and of high crystalline quality. The thickness of this layer of LiNbO3 obtained by epitaxy is also perfectly controlled. The LiNbO3 layer can be formed directly on substrates 10 of different sizes, without an intermediate transfer step. This advantageously makes it possible to reduce the manufacturing costs of such a stack of layers 10, 21, 30. Such a method can also be directly integrated into a production plant with standard CMOS technology (MOS transistors - Metal/Oxide/ Semiconductor - N- and P-type complementary).

Substrate 10 can be a solid silicon substrate, called bulk. Alternatively, this substrate 10 can be an SOI type substrate (acronym for “Silicon On Insulator” meaning silicon on insulator). Other substrates 10 can be envisaged, for example substrates based on SiC. Such substrates have full compatibility with silicon technologies for microelectronics. The silicon can also be a thermal and/or electrical conductor. This makes it possible, for example, to evacuate the heat generated in a BAW filter in operation. This possibly makes it possible to contact the device via the rear face 100, in particular when the silicon is doped.

The first electrode or nucleation layer 21 is preferably based on a refractory transition nitride. It preferably has a hexagonal or face-centered cubic crystallographic structure. Such a structure notably allows epitaxy of the nucleation layer on silicon oriented along (001) and (111). This also makes it possible to epitaxial an LN/LT material according to different crystalline orientations. Preferably, the silicon is oriented along (111). The nucleation layer 21 is for example based on titanium nitride TiN. TiN is typically a thermal and electrical conductor. TiN is furthermore a rigid material making it possible to confine an acoustic wave in an upper layer of the LN/LT type. The TiN can be raw or deposited on silicon in a known way. Vanadium Nitride, Zirconium Nitride, Tantalum Nitride, Hafnium Nitride, and Niobium Nitride exhibit similar properties to titanium nitride. They can also be advantageously chosen as the first electrode/nucleation layer 21. In what follows, due to its dual function of electrode and nucleation layer nucleation, the first electrode 21 is called “nucleation electrode 21”.

The nucleation electrode 21 can be formed by a physical or chemical deposition technique, for example and preferably by pulsed laser ablation PLD (acronym for “Pulsed Laser Deposition”). It can alternatively be formed by one of the following techniques: chemical vapor deposition (CVD, acronym for chemical vapor deposition), preferably chemical vapor deposition with metal-organic precursors (MOCVD, acronym for metal organic chemical vapor deposition) , PVD sputtering, plasma-enhanced atomic layer deposition (PEALD, acronym for plasma enhanced atomic layer deposition).

It is preferably epitaxied on the substrate 10. This makes it possible to obtain a monocrystalline nucleation layer 21 . According to another embodiment, the nucleation electrode 21 is polycrystalline with, for example, a preferential orientation. An orientation along a (111) growth plane can typically be chosen for a TiN nucleation electrode 21.

The nucleation electrode 21 formed on the substrate 10 is preferably stoichiometric. It has a thickness e2 of between 10 and 1000 nanometers, for example of the order of 100 nm. The thickness e2 of the nucleation electrode 21 can be chosen according to the desired crystalline quality.

Piezoelectric layer 30 LN/LT is based on lithium niobate LiNbO3 or lithium tantalate LiTaO3, or even an alloy of the Li(Ta,Nb)O3 type. This layer 30 can be formed by a physical or chemical deposition technique, for example and preferably by PLD pulsed laser ablation. It can alternatively be formed by one of the following techniques: chemical vapor deposition (CVD, acronym for chemical vapor deposition), preferably chemical vapor deposition with metal-organic precursors (MOCVD, acronym for metal organic chemical vapor deposition) , PVD sputtering, molecular beam epitaxy (MBE, acronym for Molecular Beam Epitaxy).

The LN/LT piezoelectric layer 30 is advantageously epitaxied on the nucleation electrode 21. It can be monocrystalline or polycrystalline, with for example a preferential orientation. An orientation according to a growth plan (0001) can typically be chosen. This makes it possible to maximize the speed of propagation of the acoustic waves within the LN/LT layer. Other orientations can be chosen depending on the desired application.

The LN/LT layer 30 formed on the nucleation electrode 21 is preferably stoichiometric, for example Li,Nb,O ₃ . The atomic percentage of lithium is ideally close to 50%. The LN/LT layer 30 has a thickness e3 of between 50 nanometers and a few hundred nanometers or a few microns, for example of the order of 500 nm. The thickness e3 of the LN/LT layer 30 can be chosen according to the desired resonance frequency.

According to one possibility, the nucleation electrode 21 and the LN/LT layer 30 are both produced in situ by PLD in the same growth reactor. The growth of the LN/LT layer 30 can thus be carried out directly after the end of growth of the nucleation electrode 21. 30LN/LT. The surface of the nucleation electrode 21 is therefore neither contaminated, nor polluted, nor oxidized. This avoids an intermediate cleaning step. The duration of the process is thus reduced. This also makes it possible to limit the appearance of roughness during the formation of the LN/LT layer. The state of the interface between the LN/LT layer 30 and the nucleation electrode 21 is therefore not modified and the surface of the LN/LT layer 30 is thus optimized.

The second electrode 22 is typically based on an electrically conductive electrode material. According to one possibility, this electrode material may also have a high acoustic impedance. Platinum, aluminum or molybdenum can be chosen as the electrode material. The second electrode 22 can be structured so as to present electrode patterns. Such an electrode 22 is typically formed by lithography/etching from an electrically conductive layer deposited on the LN/LT layer. In the electro-acoustic devices 1 according to the present invention, the piezoelectric layer 30 is sandwiched between the first and second electrodes 21, 22, in the vertical direction z. The piezoelectric layer 30 is preferably directly in contact with the first and second electrodes 21, 22.

There picture 2 illustrates another embodiment of device 1. In this example, device 1 is an FBAR type BAW filter. The characteristics of the various elements described above in the case of the BAW HBAR (substrate, nucleation electrode, piezoelectric layer, second electrode) are also valid in the embodiments which follow, mutatis mutandis. In addition to the elements described above, the BAW FBAR comprises a cavity 40 formed in the substrate 10, under the piezoelectric layer 30 and the nucleation electrode 21.

This cavity 40 can be formed by etching the substrate 10 through an opening of the nucleation electrode 21. Advantageously, in the case of a substrate silicon and a TiN nucleation electrode 21, the S _Si :S _TiN etching selectivity between Si and TiN is sufficiently high, typically greater than 5:1, so that the nucleation electrode 21 does not is not damaged during the etching of the silicon. Silicon etching can be done by wet etching using a dilute ammonia solution NH4OH or tetramethylammonium hydroxide (TMAH). Nucleation electrode 21 therefore forms, with piezoelectric layer 30, a membrane suspended above cavity 40.

The cavity 40 is preferably formed directly above the piezoelectric layer 30. It can extend on either side of the edges 31, 32 of the piezoelectric layer 30, in projection in the xy plane.

THE Figures 3A, 3B illustrate different embodiments of a device 1 of the BAW-SMR type. In these devices, in addition to the elements of the BAW HBAR, a stack 50 of layers 51, 52 forming an “acoustic” Bragg reflector is inserted between the substrate 10 and the nucleation electrode 21.

This stack 50 is preferably epitaxied on the substrate 10. It preferably exhibits electrical conduction through all of the layers 51, 52. In the embodiment illustrated in Figure 3A , the electrical contacts 41, 42 are made on the same side of the device 1, typically on the front face 101, directly on the first and second electrodes 21, 22. In the embodiment illustrated in Figure 3B , the electrical contacts 41, 42 are made on two opposite sides of the device 1, typically on the front face 101 on the electrode 22 and on the rear face 100 of the substrate 10. The first electrode 21 is then indirectly contacted via the substrate 10 and the stack 50. This makes it possible to avoid a resumption of contact on the front face for the first electrode 2, which is located under the piezoelectric layer 30. The number of process steps is thus reduced. This advantageously makes it possible to increase the exploitable surface of the piezoelectric layer 30 on the front face, in the xy plane.

The layers 51, 52 are respectively layers of high acoustic impedance and of low acoustic impedance, relative to each other. A factor greater than 2, and preferably greater than 3, will typically be chosen between the values of high acoustic impedance and of low acoustic impedance (Mrayl unit). Advantageously, the layers 51, 52 can be electrically conductive. The high acoustic impedance layers can be made of HfN or TaN for example. The low acoustic impedance layers can be TiN or VN for example. The last layer 52 of the stack 50 under the piezoelectric layer 30, for example in TiN, can advantageously form the electrode of nucleation 21. This further reduces the number of steps needed to manufacture the BAW-SMR.

THE figures 4A to 4E illustrate steps of a method of manufacturing a BAW-FBAR, according to one embodiment of the invention.

As shown in figure 4A , prior to the formation of the nucleation electrode 21, the silicon substrate 10 can be structured so as to form a mesa structure of height e1, bordered by at least one recess of material 11. The height e1 of the mesa structure can be of the order of a few hundred nanometers to a few microns, for example of the order of 500 nm. This structuring is typically done by conventional lithography/etching.

As shown in figure 4B , the nucleation electrode 21 is formed, preferably by epitaxy, on and around the mesa structure. A planarization step, typically by CMP chemical mechanical polishing, can then be performed.

As shown in Fig. 4C , the piezoelectric layer 30 is then epitaxied on the nucleation electrode 21, then the second electrode 22 is formed, for example by PVD deposition, on the piezoelectric layer 30.

As shown in 4D figure , the stack of layers 30, 22 is structured by lithography/etching so as to have a lateral dimension I2 smaller than the lateral dimension I1 of the underlying mesa structure. Advantageously, the nucleation electrode 21 has sufficient etching resistance to obtain good control of the etching stoppage during this first etching, at the surface of the nucleation electrode 21. According to one embodiment, when the Nucleation electrode 21 is made of TiN, it forms a stop layer for etching.

As shown in Figure 4E , a second etching can be performed to form at least one opening 41 in the nucleation electrode 21, for example at the edge of the stack of layers 30, 22. The layers 30, 22 are typically protected by the resin mask encapsulating the layers 30, 22, during the opening of the nucleation electrode 21. A third etching then makes it possible to form the cavity 40 under the stack of the layers 30, 22, 21. In particular, the mesa structure of the substrate silicon is removed through opening 41 during this third etching. It is not necessary to close the cavity 40. The opening 41 in the layer 21 is not necessarily filled or resealed.

Preferably, the first etching makes it possible to etch the piezoelectric layer 30 selectively with respect to the nucleation electrode 21. The third etching enables substrate 10 to be etched selectively with respect to nucleation electrode 21.

An FBAR-type device 1 BAW is thus advantageously obtained. The present invention advantageously makes it possible to form electro-acoustic devices comprising an LN/LT piezoelectric layer of good crystalline quality on silicon-based substrates, by means of a nucleation electrode made of an electrically conductive refractory material based on nitride, typically based on transition nitride. These BAW electro-acoustic devices are advantageously directly integrable or co-integrable in silicon technology. Other applications can be considered. The invention is not limited to the embodiments previously described.

Claims (11)

Method for producing an electro-acoustic device (1) comprising a piezoelectric layer (30), said electro-acoustic device (1) being a bulk acoustic wave filter guided on a Bragg reflector, said method comprising the following steps: • Provide a silicon-based substrate (10), • Form on the substrate (10) a first electrode (21), • Form the piezoelectric layer (30) on the first electrode (21), • Form on the piezoelectric layer (30) a second electrode (22), in which the first electrode (21) is chosen from an electrically conductive refractory material based on nitride and in which the formation of the piezoelectric layer (30) takes place by epitaxy, on the said first electrode (21), of a material of the ABO3 type, O being oxygen, A being at least one first chemical element taken from sodium (Na), potassium (K), barium (Ba), lithium (Li), lead (Pb), and B being at least a second chemical element taken from zirconium (Zr), titanium (Ti), niobium (Nb), tantalum (Ta),
said method further comprising, before forming the first electrode (21), forming a stack (50) of layers (51, 52) having alternately high and low acoustic impedances, the low impedance layers (52) being based on TiN or VN, and the high impedance layers (51) being based on HfN or TaN. Process according to the preceding claim, in which the refractory electrically conductive material based on a nitride is taken from refractory transition nitrides based on a transition metal. Process according to the preceding claim, in which the nitride-based refractory electrically conductive material is chosen from among TiN, VN, ZrN, TaN, HfN, NbN, or their alloys. Method according to any one of the preceding claims, in which the electro-acoustic device (1) is a bulk acoustic wave filter of the suspended membrane type, the said method further comprising, after formation by epitaxy of the piezoelectric layer (30) , an etching of the silicon-based substrate (10) under the first electrode (21) bearing the piezoelectric layer (30), so as to form a cavity (40). Process according to any one of the preceding claims, in which the formation of the first electrode (21) and the formation of the piezoelectric layer (30) are carried out by successively pulsed laser ablation within the same reactor without venting. between these formations. Process according to any one of the preceding claims, in which the substrate (10) is based on silicon oriented along (111), the first electrode (21) is based on titanium nitride TiN oriented along (111), and the layer piezoelectric (30) is oriented according to (0001). Electro-acoustic device (1) comprising, stacked in a so-called vertical direction (z), a silicon-based substrate (10), a first electrode (21) on said substrate (10), a piezoelectric layer (30) on said first electrode (21), said piezoelectric layer (30) being based on a material of the ABO3 type, O being oxygen, A being at least one first chemical element taken from sodium (Na), potassium (K ), barium (Ba), lithium (Li), lead (Pb), and B being at least a second chemical element taken from zirconium (Zr), titanium (Ti), niobium (Nb), tantalum (Ta), a second electrode (22) arranged on the piezoelectric layer (30), the device (1) being characterized in that the first electrode (21) is made of an electrically conductive refractory material based on nitride,
said electro-acoustic device being a bulk acoustic wave filter guided on a Bragg reflector, comprising, between the substrate (10) and the first electrode (21), a stack (50) of layers (51, 52) presenting alternately high and low acoustic impedances, the low impedance layers (52) being based on TiN or VN, and the high impedance layers (51) being based on HfN or TaN. Device (1) according to the preceding claim, in which the electrically conductive refractory material based on nitride is taken from refractory transition nitrides based on a transition metal, for example TiN, VN, ZrN, TaN, HfN, NbN, or their alloys. Device (1) according to any one of claims 7 to 8, in which the electro-acoustic device (1) is a bulk acoustic wave filter of the suspended membrane type, comprising a cavity (40) under the first electrode ( 21) carrying the piezoelectric layer (30). Device (1) according to any one of claims 7 to 9, in which the first and second electrodes (21, 22) are contacted at a front face (101) of the device (1). Device (1) according to any one of claims 7 to 9, in which the first electrode (21) is contacted at a rear face (100) of the device (1), and in which the second electrode (22) is contacted at a front face (101) of the device (1).

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